There are several alternatives for providing power to electrical systems implemented within a rotating reference frame (e.g., the rotating reference frame of a wheel). The electrical power may be commutated from a stationary reference frame (e.g., a vehicle), however, this requires multiple different implementations to accommodate the different geometries and configurations of various vehicle and wheel combinations. Batteries provide an alternative for supplying power, but replacing or recharging batteries, even on an infrequent basis, may be undesirable.
A third option is to generate electrical power within the rotating reference frame of the wheel. A number of schemes have been devised to independently generate electrical power on the wheel of a vehicle. An independent source of electrical power on the wheel provides many advantages. For example, emergency lighting or other safety features can be more readily and reliably effected with an independent, on-wheel, power source.
Schemes to realize such a power source have typically taken the form of an inertia-based, on-wheel generator that derives energy from the rotation of the wheel.
Examples of such schemes include the following.
U.S. Pat. No. 4,229,728 to Tremba discloses a generator on a vehicle wheel. The generator is composed of a first member and a second member. The first member is mounted coaxially with the wheel's axis of rotation and contains a permanent magnetic element. The second member contains an inductor and is mounted to the wheel so that is can be rotated proximate to the first member. The second member rotates free of the first member and has a mass center which is displaced from the axis of the first member, resulting in a pendulum or vertical position seeking affect. The difference in angular velocities is used to generate electricity.
U.S. Pat. No. 4,539,496 to Thomas discloses a generator attached to one end of an elongated arm, the other end of which is attached to a rotationally free structure that is coaxial with the axis of the wheel. The rotationally free structure allows the wheel to rotate without causing the elongated arm to rotate. The generator has a drive wheel that attaches to its axis. This drive wheel engages the surface of the wheel. Therefore as the wheel turns the generator axis turns and electrical energy is generated. The generator and additional counterweight supplies a torque, which prevents the elongated arm and attached mechanisms from rotating with the wheel.
U.S. Pat. No. 4,775,919 Pearsall discloses a means of generating power as part of a hubcap lighting system. Two housings are implemented within the hubcap. The first housing contains the armature coils, and is attached to and rotates with the wheel. The second housing contains the magnets, and is attached to the center axis of the hubcap and rotates freely with respect to the first housing. Power is generated by the difference in velocity of the two housings. A counterweight to the second housing is implemented to increase resistance to rotation.
Such schemes are disadvantageous in that access to the wheel's axis of rotation is required. This drawback is addressed in U.S. Pat. No. 4,429,232 to Thomas et al., which discloses a generator characterized by a circular track attached to the relieved circumferential surface of a wheel, a carriage coupled to the track by rotary bearings, a generator supported by the carriage, and a clutch wheel coupling an armature of the generator to the track. The track, which preferably has a T shaped cross section, has a driving surface which causes the clutch wheel to rotate when there is relative motion between the track and the carriage and/or between the carriage and the air within the tire. The generator, then, is not subjected to centrifugal forces, but only to normal gravitational forces.
An additional disadvantage of typical prior art schemes is that they are subject to a rollover state. The rollover state is the condition in which a counterweight of the generator's stator is carried over a critical angle and begins to rotate with the wheel.
This disadvantage is recognized and addressed with some success by U.S. Pat. No. 4,761,577 to Thomas et al., which discloses a generator contained within a wheel-mounted hub. The generator is pivotally supported in arms of a yoke. The yoke and generator have an offset center of gravity, tending to give the yoke and generator a pendulum-like behavior within the housing. During wheel rotation, the generator tends to remain along a gravitational line of force, forcing a small contact wheel, associated with the generator, to turn against the hub.
At high speeds, the generator assembly tends to rotate with the wheel due to electromagnetic torque coupling between the small contact wheel and the hub on which it turns. Such rotation of the generator assembly with the wheel can lead to the rollover state.
To address the situation a current limiter switch is implemented for the generator such that if the axis of the small contact wheel exceeds a specified angle, the switch is activated and the excitation current of the stator ceases. When the stator is not sufficiently excited, the generator is disabled, thus minimizing the friction between the rotor and the stator. In this condition the generator tends to fall toward a gravitational line of force.
This scheme addresses one way of attempting to avoid entering the rollover state when such state is approached gradually (i.e., smooth road surface). In many cases the rollover state is entered suddenly, for example, driving over a pothole in the road may cause a sudden transition to the rollover state. This scheme does not address methods for terminating the rollover state once it has been achieved. Moreover, this scheme, like many prior art schemes requires access to the wheel's axis of rotation.